Magnetic lens



Mrch 6, 1945.

J. HILLIER MAGNETIC LENS Jriginal Filed Oct. 16, 1940 2 Sheets-Sheet lfla. 1.

ISnventor James Hill/lie r -c Q"/ Gttomeg .1. HILLIER MAGNETIC LENSMarch 1945.

2 Sheets-Sheet 2 Original Filed Oct. 16, 1940 a n M 5 wm w v 6/ 6\ 3nnentor James Htllzer Gtforneg Patented Mar. 6, 1945 MAGNETIC LENS JamesHillier, Collingswood, N. J., assignor to Radio Corporation of America,a corporation of Delaware Original application October 16, 1940, SerialNo. 361,348. Divided and this application January 15, 1942, Serial No.426,865?

2 Claims.

This case is a division of copending application Serial No. 361,348,filed October 16, 1940, now U. S. Patent 2,292,877.

This invention relates to magnetic lenses of the type used in electronmicroscopes and the like and has for its principal object to provide animproved magnetic lens of the character described and one wherein thepole pieces are maintained in rigid permanent alignment.

It has been shown that a. magnetic field may be used to focus electronrays in a manner analogous to the focusing of light rays by an opticallens. By a suitable arrangement of lenses, a

magnified image of an object may be formed by.

electron rays passing through the object. Magnifications of the order oftwenty or more thousand times have been used. Magnifications of thatorder require extremely accurate lenses if objectionable aberrations areto be avoided.

The pole pieces of magnetic lenses should possess axial symmetry and theaxes should be collinear. These conditions may be best met by turningallthe active surfaces of the pole pieces in the manner hereinafterdescribed.

The invention will be described by referring to the accompanyingdrawings, in which Figure 1 is a sectional view of a preferredembodiment of a magnetic lens; Figs. 2, 3, 4 and 5 are sectional viewsillustrating the method of machining the pole pieces; Figure 6 is asectional view of a modified magnetic lens; and Figures '7, 8, 9 and 10are sectional views illustrating a modified method of machining the polepieces.

Referring to Fig. 1, a. magnetic lens is formed by surrounding asolenoid winding l with a magnetic shell 3. The shell includes a baseplate 5 which has an inwardly extending coaxial tubular portion 1, acylindrical outer shell 9 and an annular top plate I 4. The parts of theshell are preferably held together by drive fits; or mechanicalfastening means, such as screws or threaded parts, may be used. The lensis completed by pole pieces it, which include a pair of symmetricalmembers, maintained in spaced relation by a non-magnetic tubular spacerI5 to form a unitary structure. The shell, which is made of magneticmaterial, and the pole pieces are held together by a drive fit or anysuitable means.

The pole pieces are machined from a single blank which is secured to.the clutch jaws IQ of a lathe, automatic screw machine or the like. Thefirst operation is to turn the blank to form the outer walls 2|,inwardly extending sloping side walls 23, sloping top wall 25 and fiattop wall 21. The second operation is to bore a hole 29 along the axis 3|to a depth not less than the final length of the inner cylindrical wallsof the pole piece. The third step is to shrink and insert within saidhole the non-magnetic retainer or spacer l5 so that it forms a tight fitupon expansion. The non-magnetic retainer is drilled along its axis toform an opening corresponding to the finished inner cylindrical walls ofthe pole pieces. The blank after the completion of the third step isshown in Fig. 3. The fourth step is to divide the magnetic portions ofthe pole pieces by cutting the gap 33 (see Fig. 4) from the sloping sidewalls 23 to the outer portion of the non-magnetic tubular retainer I5.The fifth and final step is to cut from the outer wall 2| inwardly toform the lower fiat bottom Wall 35 (see Fig. 5) and the bottom inwardlysloping wall 37.

It should be understood that the invention contemplates a reversal ofparts; i. e., the nonmagnetic retainer may surround the pole piecesinstead of having the pole pieces surround the retainer. If the partsare thus reversed, the lens will appear as shown in Fig. 6 in which asolenoid coil 4| is surrounded by a shell 43 of magnetic material. Theshell includes a. base plate 45, an inwardly extending tubular portion41, an outer cylindrical member 49, and an annular top plate 5|. Thepole pieces 53, are tightly fitted between the top plate 5| and thetubular portion 41.

The method of machining the pole pieces will be described by referringto Figs. 7, 8,9 and 10. A blank 51 is fastened to a lathe or the like bymeans of chuck Jaws 59. The blank is machined to form the outercylindrical wall 6|, including inwardly sloping portion 63. The inwardlysloping portion 63 is extended by cutting parallel to the flat plate 65to form a gap 69. The end of the blank is cut to form a flat plate 65and an inwardly sloping portion 61. The next step is to fit tightly a.non-magnetic tubular retaining member H around the cylindrical wall 6|.The retainer is preferably shorter than the cylindrical wall so that themagnetic portion of the upper pole piece 53 contacts the annular plate'5! to complete the magnetic circuit.

The blank is bored along its axis to form the inner cylindrical surfaces13 of the pol pieces. The next steps are to cut the blank at the lowerportion of the pole pieces to form a bottom wall 15 parallel to theannular top plate 65 and finally to cut the inwardly sloping portion 11.

In both the present method and the first described or preferred methodthe separating of the pole pieces from the blank may be done by clampingthe pole pieces in a collet or other chuck so that the bottom wall andsloping wall portion may be machined from the front instead or the-,rear. Any slight inaccuracy caused by thus removing the piece from theoriginal position for the cutting on operation will have no observableefiect on the lens because the lens field is essentially located betweenthe inner cylindrical walls,

and adjoining Parallel walls.

Since all of the foregoing operations may be performed without machine,it follows that the pole pieces may be made symmetrical, and collinear.Furthermore,

the foregoing steps are simple turning operations which can be performedaccurately and inexpensively. The non-magnetic tubular portion retainsthe pole pieces in the desired alignment without affecting the field andmakes the pole pieces a unitary structure.

What is claimed is:

1. A unitary demountable pole piece structure nently spaced collinearand symmetrical relation.

removing the blank from the I 2. A-unitary demountable pole piecestructure for a magnetic lens including an upper magnetic pole piece anda lower magnetic pole piece, said polepieces having concentric inner andouter walls-and a tubular non-magnetic retaining member intimatelyjoined to said outer walls, said member comprising the sole support formaintaining said magnetic pole pieces in permanently spaced collinearand symmetrical relation.

JAMES mum.

